(a) Field of the Invention
The present invention relates to an electric ballast system, and more particularly, to a control circuit for an electric ballast system.
(b) Description of the Related Art
Electric ballast systems perform the initial driving of a lamp and thereafter supply stable power to the lamp. The conventional electric ballast system is typically comprised of a DC-DC converter for supplying DC power, and a lamp driving circuit for controlling the driving of a lamp. In such a conventional ballast system, power is supplied to the lamp driving circuit from a point at which an upper switch and a lower switch of the DC-DC converter meet. That is, power is supplied from this point without passing through the lamp.
However, the supply of power to the lamp driving circuit in the manner described above has many drawbacks. In particular, during a soft start or dimming, driving frequencies of the lamp quicken and a dead time decreases. At this time, a capacitor for supplying power to the lamp driving circuit limits the dead time operation. That is, a voltage of the point between the switches does not become zero during switching as a result of the capacitor connected between the upper and lower switches of the DC-DC converter. Hence, zero voltage switching is not able to occur, a temperature of a switching MOSFET increases, and normal lamp operation becomes difficult.
Further, since power is supplied to the lamp driving circuit without passing through the lamp, if there is no bulb in the lamp, power is continuously supplied to the lamp driving circuit, causing the switching MOSFET to burn out. To prevent such a problem, a protection circuit must be additionally installed in the lamp driving circuit.